1. Field of the Invention
The invention relates to a process for regenerating absorbent solutions used for removing gaseous impurities from gaseous mixtures by stripping with steam. The invention relates in particular to the regeneration of absorbent solutions used for removing CO.sub.2, H.sub.2 S, HCN, SO.sub.2 and other acidic gases from gaseous mixtures. The process of the invention may furthermore be applied to other cyclic purifying processes for removing acetylene, methane, ethane, benzene, toluene and other hydrocarbons as well as other gaseous impurities as far as the known absorbent liquids suited for this purpose can be regenerated by stripping with steam.
2. Description of the Prior Art
In the known cyclic purifying processes, an absorbent solution or an absorbent liquid in general is circulated between an absorption phase and a regeneration phase. In the absorption phase the absorbent solution is contacted with the gaseous mixture to be purified, the gaseous impurities contained therein being removed. in the regeneration phase the abosrbent solution is treated with steam in order to desorb the impurities previously absorbed.
The best-known purifying processes of this kind concern the absorption of gaseous impurities such as CO.sub.2, H.sub.2 S, HCN, SO.sub.2 and/or similar gases by using alkaline aqueous solutions. Examples of absorbent solutions are alkali carbonate solutions which are optionally activated by adding As.sub.2 O.sub.3, glycine or other amino acids or by adding ethanol amines such as monoethanol amine, diethanol amine or similar compounds, or solutions of alkali metal salts of amino acids, ethanol amine solutions such as solutions of monoethanol amine, diethanol amine, triethanol amine or similar compounds such as solutions of alkali phosphates, alkali phenates, alkali borates and similar compounds. The absorption of SO.sub.2 is carried out in particular by using alkali sulfite/alkali bisulfite solutions.
The afore-mentioned purifying processes can be carried out at low temperatures, i.e., at about normal temperature, or at elevated temperatures. In the case of the so-called "conventional cycle" (see Kohl and Riesenfeld, "Gas Purification" (1960) p. 24), the absorption is carried out at temperatures near ambient temperature, and the regeneration at the boiling temperature of the absorbent solution and at pressures near atmospheric pressure. In the case of this conventional cycle, a heat exchanger is provided in addition to the absorption and regeneration device, by means of which the spent absorbent solution is heated by the regenerated solution. Furthermore a cooler is provided and arranged before the absorption phase.
In particular, when removing CO.sub.2 the absorption can also be carried out in heat, i.e., at temperatures of from about 90.degree. to 120.degree.C, which is near the regeneration temperature. In this case the so-called "isothermic cycle" or "optimal cycle" is used (see Kohl and Risenfeld, loc. cit. p. 134,) the use of the heat exchanger employed in the conventional cycle superfluous. Although this means a facilitation, the purification of the gaseous mixture to the values necessary in modern, large-scale practice is difficult at the high absorption temperature and generally necessitates a purifying cycle consisting of two steps (see U.S. Pat. No. 1,971,798).
All purifying processes mentioned show the disadvantage of a low thermal efficiency. In the processes best known and best capable of being improved for removing CO.sub.2, the heat consumption is about 1000 to 1300 kcal/m.sup.3 of CO.sub.2. This heat consumption appears the higher the more it is considered that the heat consumption in cycles involving regeneration with air is only about 450 to 600 kcal/m.sup.3 of CO.sub.2, even in two-stage plants with a purification degree of from about 0.02 to 0.05 percent of CO.sub.2).
The regeneration cycle involving air and other desorbent inert gases is carried out on large scale such that the absorbent solution leaving the absorber and having a temperature of from about 75.degree. to 80.degree.C is gradually heated by external heat to temperatures of from about 85.degree. to 90.degree.C and subsequently passed to the regenerator in countercurrent with the desorbent gas. The absorbent solution is regenerated and simultaneously cooled to temperatures of from 70.degree. to 75.degree.C. Finally the absorbent solution is returned to the absorber to complete the cycle.